1. Field of the Invention
The present invention relates to an antenna device and a method for manufacturing the antenna device.
2. Description of the Related Art
Common automobile antenna devices are amplitude modulation/frequency modulation (AM/FM) radio antenna devices capable of receiving AM broadcasts and FM broadcasts. Although AM/FM radio antenna devices of a rod type have been used, AM/FM antenna devices of a shark fin type have been preferred in recent years because of attractive design. The antenna device of a shark fin type is named after its external shape.
Antenna devices of a shark fin have an upper limit of 70 mm in height and an upper limit of about 200 mm in length to comply with legal regulations.
Low-profile antenna devices due to the regulation have disadvantages, such as low impedance or radiation resistance and thus low receiving sensitivity.
For example, Japanese Patent No. 5237617 discloses an antenna device that solves such disadvantages.
The device of Japanese Patent No. 5237617 includes an umbrella antenna component and is called a capacity loaded or top loaded antenna. The feeding point of the antenna component is connected to the input terminal of an amplifier through an antenna coil.
The antenna device disclosed in Japanese Patent No. 5237617 includes the amplifier to compensate for the low reception efficiency in the low-profile antenna component.
However, the low-profile antenna component has low impedance or radiation resistance of the antenna component, resulting in reception loss due to the impedance mismatch between the antenna component and the amplifier.
Although the impedance mismatch can be reduced by providing an additional matching circuit including a resistor, a capacitor, and an inductor on the antenna component, the material and production costs increase.